Actually, maybe neither Talia or Bane thought Bruce would even find out that the child who escaped was the child of Ra's. Bruce only figured it out during an hallucination anyways if I remember correctly. He knew it was a mercenary, saw Ra's, and then heard the story and made the connection that the child was the child of Ra's. And the doctors do tell Bruce that Bane wouldn't want the story to be told, so maybe they didn't believe he would find out anyways? This, of course, is if I remember correctly.